Because of the dynamic characteristics of the delivered short lasting torque impulses there is a problem to measure the actually delivered output torque magnitude in this type of power wrenches. Due to varying frictional resistance in screw joints the torque level is not a definite measurement of the actually obtained clamping force anyway. So, instead of trying to determine the torque imposed on the screw joint the more reliable clamping force is determined by measuring and establishing the sum of the rotational increments imposed on screw joint by repeated torque impulses. In consideration of the pre-established angle-to-clamping-force relationship of the actual screw joint, i.e. the pitch of the screw joint thread, the sum of the rotational increments corresponds to a certain clamping force obtained in screw joint. Accordingly, the total rotational movement of the output shaft is obtained by summarizing the rotational increments can be indicated by an angle encoder provided at the impulse unit of the power wrench.
In practice this has been done by measuring the rotational increments of the inertia drive member of the impulse unit in relation to the power wrench housing and forming a sum of these increments. However, this gives a correct information of the amount of rotation imposed on the screw joint being tightened only if the power wrench housing remains absolutely immobile during the tightening operation. If however some rotational displacement of the power wrench housing occurs the angle measurement in the power wrench impulse unit would be misleading and not correspond to the actually obtained rotational movement of the screw joint. This means that the sum of the rotational increments measured in the impulse unit gives misleading information of the tightening level actually obtained in the screw joint.
So, there is a problem to verify the true amount of rotation imposed on the screw joint, because manually supported tools of this type are dependent on the operator's ability to keep the wrench housing immobile during torque delivery. In some cases the impulse wrench may tend to rotate somewhat in the opposite direction due to reaction forces transferred to the wrench housing during torque delivery. In other cases the operator, consciously or not, may rotate the wrench housing in the tightening direction which also makes the rotational increments indicated by the angle encoder of the wrench misleading as to the actual rotational movement imposed on the screw joint being tightened.
This means that in cases where the operator does not succeed to keep the power wrench housing immobile, i.e. to prevent rotational displacements of the housing during tightening, the data obtained from the angle encoder of the impulse mechanism will not give a true information on the obtained screw joint rotation and, hence, the tightening level of a screw joint.